1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer using electrophotography, and particularly relates to an image forming apparatus capable of forming a full color image using a belt-like intermediate transferor.
2. Description of the Related Art
JP-B-2,962,088, JP-A-2001-134040, JP-A-2003-195712 and JP-A-2003-215877 are referred to as related arts.
In the background art, various systems have been proposed and manufactured as this kind of image forming apparatus such as copying machines or printers using electrophotography. Of the image forming apparatus, particularly color image forming apparatus for forming a full color image can be roughly classified into a type using an intermediate transferor and a type not using an intermediate transferor. In the image forming apparatus using an intermediate transferor, a toner image formed on a photoconductor is once primary-transferred onto the intermediate transferor, so that primary transfer can be performed independently of the material of a recording medium. Thus, the image forming apparatus using an intermediate transferor is advantageous to improvement in the quality of the full color image.
The color image forming apparatus using an intermediate transferor is classified into a so-called “four-cycle system” and a so-called “tandem system”. The “four-cycle system” color image forming apparatus is designed to form a color image as follows. That is, color toner images of yellow, magenta, cyan, black and the like formed in turn on a single photoconductor are primary-transferred onto an intermediate transferor so as to be superimposed on one another. After that, the toner images of yellow, magenta, cyan, black and the like multilayer-transferred onto the intermediate transferor are secondary-transferred onto a recording medium by a secondary transfer roll.
On the other hand, the “tandem system” color image forming apparatus is designed to form a color image as follows. That is, toner images of different colors from one another, such as yellow, magenta, cyan and black, formed respectively on plural (e.g. four) photoconductors are primary-transferred onto an intermediate transferor so as to be superimposed on one another. After that, the toner images of yellow, magenta, cyan, black and the like multilayer-transferred onto the intermediate transferor are secondary-transferred onto a recording medium by a secondary transfer roll.
In the color image forming apparatus using an intermediate transferor which apparatus is either the “four-cycle system” or the “tandem system”, there may occur a fluctuation in the moving velocity or the like of the photoconductor(s) or the intermediate transferor when the toner images formed on the photoconductor(s) are primary-transferred onto the intermediate transferor or when the toner images primary-transferred onto the intermediate transferor are secondary-transferred onto the recording medium. In such a case, color misregistration occurs due to the fluctuation in the velocity or the like of the photoconductor(s) or the intermediate transferor. For example, the fluctuation in the velocity of the intermediate transferor is caused by a fluctuation in the load on the intermediate transferor due to the secondary transfer roll or a cleaning unit touching or leaving the intermediate transferor.
Therefore, related art that can prevent such color misregistration from occurring due to a fluctuation in the velocity or the like of a photoconductor or an intermediate transferor have been disclosed in JP-B-2,962,088, JP-A-2001-134040, JP-A-2003-195712, JP-A-2003-215877, etc.
A color printer according to JP-B-2,962,088 includes a photoconductor rotated by a photoconductor drive unit, an exposure unit for exposing the photoconductor to laser beams to thereby form a latent image, a development unit for developing the latent image formed on the photoconductor in different color by each rotation to thereby form toner images, an intermediate transferor to which the toner images of different colors formed on the photoconductor are transferred to be superimposed on one another, a transfer unit for transferring the images transferred onto the intermediate transferor further onto a recording medium, and a control unit for controlling the photoconductor drive unit to rotate the photoconductor at a desired rotation velocity. The intermediate transferor and the photoconductor are disposed in contact with each other so that the intermediate transferor is driven to rotate by the photoconductor. The color printer further includes a detection unit for detecting the intermediate transferor passing a specific position, and an intermediate transferor rotation velocity measuring unit for obtaining the rotation velocity of the intermediate transferor on the basis of the output of the detection unit. Based on the measured rotation velocity of the intermediate transferor, the desired rotation velocity of the photoconductor is changed before the latent image is formed on the photoconductor. Thus, the difference in rotation velocity among the rotations of the intermediate transferor is suppressed. The desired rotation velocity of the photoconductor is kept constant during the formation of the latent image on the photoconductor.
Image forming apparatus according to JP-A-2001-134040 includes a latent image carrier, an exposure unit for exposing the latent image carrier plural times correspondingly to respective images of color components of a color image, a development unit for developing latent images of the respective color components formed on the latent image carrier with toners respectively, an intermediate transferor marked with a reference position mark indicating a reference position for determining the exposure start timing of the exposure unit, a transfer unit for transferring toner images of the respective color components developed on the latent image carrier onto the intermediate transferor so as to register the end portions of the toner images with one another based on the reference position, a detection unit for detecting a detection period of the reference position mark, and a control unit for obtaining a difference between the detection period of the reference position mark and a predetermined specific detection period and making control to extend/shorten the exposure start timing of each identical color component at the time of forming the next color image by time corresponding to the obtained difference.
Image forming apparatus according to JP-A-2003-195712 includes an endless photoconductor to be moved circumferentially, an exposure unit for exposing the photoconductor in an exposure position of the photoconductor based on image information to thereby form a latent image on the photoconductor, a development unit for developing the latent image with a developer in a development position downward in the moving direction of the photoconductor with respect to the exposure position of the exposure unit so as to form an image using the developer, an endless image carrier to be moved circumferentially in contact with the photoconductor in a primary transfer position downstream in the moving direction of the photoconductor with respect to the development position of the photoconductor so that the image using the developer is transferred onto the image carrier in the primary transfer position, a secondary transfer unit for transferring the image using the developer onto a recording medium in a secondary transfer position downstream in the moving direction of the image carrier with respect to the primary transfer position of the image carrier, a cleaning unit provided to be able to touch and leave the image carrier in a cleaning position downstream in the moving direction of the image carrier with respect to the secondary transfer position of the image carrier and upstream with respect to the primary transfer position so as to remove the developer remaining on the image carrier after the secondary transfer by the secondary transfer unit due to the contact operation of the cleaning unit, and a control unit for controlling the cleaning unit to perform the contact operation and the separation operation in the cleaning position when the exposure unit does not engage in performing the latent image forming operation on the photoconductor.
Color printing apparatus according to JP-A-2003-215877 accepts an input of a correction value for correcting the misregistration of a printing position with respect to a color of reference. The color printing apparatus includes a correction guide printing unit. The correction guide printing unit prints a correction guide in which grids having different widths and different intervals are disposed and superimposed on each other for the color of reference as to the printing position and a color to be corrected as to the printing position. The correction guide shows the size of printing position misregistration of the color to be corrected with respect to the reference color on the basis of a distance of a predetermined pattern expressed by the color to be corrected from a predetermined reference position. The correction guide printing unit prints a scale in which a distance from the predetermined reference position is associated with a correction value to be inputted. The scale is designed so that the correction value at the time of printing the correction guide corresponds to the reference position.
However, the aforementioned related art have problems as follows. That is, each of the related art disclosed in JP-B-2,962,088, JP-A-2001-134040, JP-A-2003-195712, etc. is designed to change the rotation velocity of the photoconductor or the intermediate transferor, to make control to extend/shorten the exposure start timing in accordance with the difference between the detection period of the reference position mark and the predetermined specific detection period, or to perform the contact operation and the separation operation of the cleaning unit in the cleaning position when no latent image is being written onto the photoconductor. On the other hand, the related art disclosed in JP-A-2003-215877 is designed so that the correction guide printing unit prints the correction guide showing the size of the printing position misregistration of the color to be corrected with respect to the reference color, and a correction value for correcting the printing position misregistration obtained by the correction guide is accepted.
However, in such image forming apparatus, an operation of the cleaning unit or the secondary transfer roll touching/leaving the surface of the intermediate transferor may be inserted into the operation of forming or transferring toner images of respective colors such as yellow, magenta, cyan and black. In such a case, there is a problem that there may occur a fluctuation of velocity in the intermediate transferor, causing color misregistration.
In the case where the contact/separation of the cleaning unit is designed to be performed when no latent image is being written onto the photoconductor, there is a problem as follows. That is, the timing when termination of the operation of cleaning the intermediate transferor by the cleaning unit is so late that the productivity in forming images per unit time is lowered inevitably.
On the other hand, according to the related art disclosed in JP-A-2003-215877, misregistration of a printing position at the time of normal image formation can be obtained visually and corrected using the correction guide. However, the operation of the cleaning unit or the secondary transfer roll touching/leaving the surface of the intermediate transferor may be inserted into the image forming operation. In such a case, there is a problem that the occurrence of color misregistration in the process direction cannot be detected.